1. Field of the Invention
The present invention relates to a magnetic random access memory (MRAM: Magnetic Random Access Memory) in which a memory cell array is comprised of storage elements using a magneto-resistance effect (Magneto Resistive).
2. Description of the Related Art
A magnetic random access memory using a tunneling magneto-resistance effect (TMR: Tunneling Magneto Resistive) is disclosed in, for example, [Roy Scheuerlein et al. “A 10 ns Read and Write Non-Volatile Memory Array Using a Magnetic Tunnel Junction and FET Switch in each Cell”, ISSCC2000 Technical Digest, pp. 128–129]. The magnetic random access memory is characterized by storing data by a magnetized state of an MTJ (Magnetic Tunnel Junction) element.
An MTJ element showing a TMR has, for example, as shown in FIG. 1, a structure in which a tunnel insulating layer is sandwiched by two ferromagnetic layers. The MTJ element can take two states, and one is a parallel state in which the directions of the residual magnetizations of the two ferromagnetic layers sandwiching the tunnel insulating layer are the same, and the other one is an anti-parallel state in which the directions of the residual magnetizations of the two ferromagnetic layers sandwiching the tunnel insulating layer are opposite to one another.
As shown in FIG. 2, when the MTJ element is made to be in a parallel state, the tunneling resistive value of the MTJ element is made to be a minimum. Suppose that this state is, for example, a “0” state. On the other hand, as shown in FIG. 3, when the MTJ element is made to be in an anti-parallel state, the tunneling resistive value of the MTJ element is made to be a maximum. Suppose that this state is, for example, a “1” state.
Here, in order to prevent both of the two ferromagnetic layers from being inverted when switching of the magnetized states of the MTJ element (magnetization inversion) is carried out, a given difference is provided between the coercive forces of the two ferromagnetic layers. Accordingly, only the magnetization of the ferromagnetic layer having a smaller coercive force is inverted, and the above-described parallel state and anti-parallel state can be realized.
The ferromagnetic layer having a smaller coercive force among the two ferromagnetic layers is called a memory layer (free layer), and the ferromagnetic layer having a larger coercive force is called a fixed layer (pinned layer). As a method for providing a difference between the coercive forces, for example, there are methods in which the materials are made different from each other, the volumes are made to have a difference therebetween, and the like. However, as the most general method, there is a method in which the magnetized state of the fixed layer is fixed by combining the antiferromagnetic layer with the fixed layer.
With respect to the switching of the magnetized states of the MTJ element, the writing selectivity is important. Namely, it is important that magnetization inversion on the memory layer is executed for a selected MTJ element which will be a writing object, and magnetization inversion on the memory layer is inhibited for unselected and half-selected MTJ elements which will be not writing objects.
In particular, because error writing is easily brought about with respect to a half-selected MTJ element to which only a magnetic field in the easy or the hard axial direction is applied, for the purpose of commercial using, there has been an important object that the technique of effectively preventing error writing is proposed for.
Further, on the other hand, with respect to switching of the MTJ element, there has been requested that a write current is reduced (electric current consumption-lowering) by carrying out magnetization inversion at a small switching magnetic field due to the coercive force of the memory layer being made little. However, when the coercive force of the memory layer is made little, error writing is easily brought about in a half-selected MTJ element.